A general trend in both storage and electronics has been to make devices smaller and faster. The same trend exists with magnetoelectronic “spintronic” devices, wherein the magnetic regions are becoming ever smaller. Such devices rely, in some cases, on the switching of a region of magnetic material between two or more stable configurations of magnetization, and in other cases (e.g., magnetoresistive field sensors), on biasing of the magnetization away from a single equilibrium configuration by, and in some proportion to, the field being sensed. As magnetic devices are made smaller, and designed to switch faster, the challenge of getting the switched region to relax with certainty to a desired magnetization configuration arises. However, the conducting ferromagnetic materials used in these devices, typically transition metal alloys, have very small intrinsic magnetization damping.
While a switching of the magnetization configuration of a device, e.g., by an externally applied magnetic field or by spin injection, ideally changes the magnetic configuration without any unwanted motion of the magnetization, in practice this rarely occurs. The magnetic region of the device typically precesses rapidly with a large but gradually decreasing amplitude following switching. During the precession period, particularly during the initial stages where the amplitude is large, spin-waves, both thermally excited and those excited by the switching process and defects in the material, can interfere constructively to direct the magnetization to an equilibrium configuration other than the one intended. Even a small probability of such a spurious switching event (i.e., less than 1%) is unacceptable for memory applications. In the case of field sensors, it is also desirable to reach equilibrium quickly, i.e., to reduce the oscillatory response caused by precession of the magnetization about the new equilibrium defined by the field being sensed.
A magnetic switching device or magnetoresistive sensor may comprise a magnetic tunnel junction (MTJ) device or a spin valve (SV) device. An MTJ, in a simplest form, may comprise a stack of two ferromagnetic layers separated by a tunnel barrier at a cross-point of two conductors, one of which may be a word line (WL) and the other a bit line (BL). An SV can be made by replacing the tunnel barrier in an MTJ with a conducting layer. In both cases, the resistance of the device depends strongly on the orientation of the magnetization of the two magnetic layers relative to each other. One of the two magnetic layers is often referred to as a “free” magnetic layer or as a “storage layer.” The storage layer may comprise a single ferromagnetic or ferrimagnetic layer, or a synthetic antiferromagnetic structure with more than one ferromagnetic layer separated by one or more non-magnetic spacer layers. The magnetic orientation of the storage layer can be changed by the superposition of magnetic fields generated by programming currents IWL and IBL flowing in the conductors WL and BL, respectively. The other of the two magnetic layers is often referred to as a “fixed,” “pinned” or “reference” magnetic layer. The magnetization of the fixed layer is invariant, and as such, the programming currents IWL and IBL do not change the magnetic orientation of this fixed layer. The fixed layer can also comprise a single ferromagnetic or ferrimagnetic layer or a synthetic antiferromagnet consisting of more than one ferromagnetic layer separated by one or more non-magnetic spacer layers. The logical state (e.g., a “0” bit or a “1” bit) is generally stored in the MTJ by changing the orientation of the free magnetic layer relative to the fixed magnetic layer. When both magnetic layers have the same orientation, the MTJ typically has a low resistance associated therewith, as measured between conductors WL and BL. Likewise, the resistance of the MTJ is generally high when the magnetic layers are oriented in opposite directions with respect to one another.
Another way of operating an MTJ is by active readout, wherein the MTJ consists of a storage layer and an “interrogation” layer. The storage layer stores the “state” of the memory. The state of the memory may be determined by reading the output of the device as per the two possible orientations of the interrogation layer. Both the storage and the interrogation layer may be single ferromagnetic or ferrimagnetic layers or synthetic antiferromagnetic structures, as described above.
A conventional magnetic random access memory (MRAM) generally includes a plurality of MTJ devices connected in an array configuration. Exemplary MRAM arrays include cross-point arrays, wherein each memory cell comprises a single MTJ device connected at an intersection of a word line and a corresponding bit line, and an architecture employing a plurality of memory cells, each memory cell comprising a selection transistor coupled in series with an MTJ device. The selection transistor is used for accessing the corresponding MTJ device during a read operation. MRAM circuits are discussed in further detail, for example, in W. Reohr et al., “Memories of Tomorrow,” IEEE Circuits and Devices Mag., v. 18, no. 5, p. 17-27 (Sept. 2002), the disclosure of which is incorporated by reference herein.
The magnetization dynamics of magnetic materials are affected by the properties of the magnetic materials. Magnetic material properties of particular importance for magnetic switching devices and magnetoresistive sensors include damping and coercivity. Specifically, damping is the action whereby the amplitude of magnetic precession (oscillatory response) is decreased. Coercivity is a property of a magnetic material wherein the magnetic field required to return the magnetization of a magnetic material from saturation back to zero is measured. The coercive field, Hc, can be used to approximate the magnetic field needed to switch the magnetization, i.e., the switching field.
One challenge associated with magnetic switching devices, such as those employing small area structures, is that the shape anisotropy contribution to the switching field, assuming that the small magnetic element has an aspect ratio not equal to one in the plane of the device, increases inversely proportional to the thickness of the small magnetic element (perpendicular to the plane). This effect calls for the use of higher currents to generate the switching fields and brings about increased power consumption and increased heating. It is therefore desirable to be able to contain or decrease the coercivity of small magnetic elements.
The materials used in the above devices are typically ferromagnetic transition metal alloys. These materials tend to be severely underdamped, whether in a bulk form, a thin film or a small lithographically defined element. Certain alloys have been shown to have desirable enhanced damping properties in bulk (approximated by thick film) form. For a detailed description of these alloys, see Ingvarsson et al., U.S. Pat. No. 6,452,240 “Increased Damping of Magnetization in Magnetic Materials,” (hereinafter “Ingvarsson”) the disclosure of which is incorporated by reference herein. Ingvarsson demonstrates that certain material choices provide improved damping properties of magnetoresistive devices. It would be further desirable to benefit the switching characteristics of a magnetic switching device by changing the structure of these devices, without affecting the composition of the magnetic materials.
Examples of underdamped magnetic materials include Permalloym magnetic films, a trademark of B&D Industrial & Mining Services, Inc., having the composition Ni81Fe19, which have been shown to exhibit magnetization oscillations after magnetic switching. For a detailed description of the magnetization dynamics of these Permalloy™ magnetic films, see for example, T. J. Silva et al., “Inductive Measurement of Ultrafast Magnetization Dynamics in Thin-Film Permalloy,” J Appl. Phys., v. 85, no. 11, p. 7849 (1999), and S. Ingvarsson et al., “Role of electron scattering in the magnetization relaxation of thin Ni81Fe19 films,” the disclosures of which are incorporated by reference herein.
The damping properties of films comprising pure ferromagnetic transition elements, such as nickel, iron or cobalt are known. The damping properties of these elements are characterized by damping parameters that are too small to achieve optimal switching behavior in devices. For a detailed description of the damping properties in such materials, see J. M. Rudd et al., “Anisotropic Ferromagnetic Resonance Linewidth in Nickel at Low Temperatures,” J Appl. Phys., v. 57, no., 1 p. 3693 (1985); B. Heinrich et al., “Ferromagnetic-Resonance Study of Ultrathin bcc Fe(100) Films Grown Epitaxially on fcc Ag(100) Substrates,” Phys. Rev. Lett., v. 59, no. 15, p. 1756 (1987); and Schreiber et al., “Gilbert Damping and g-Factor in FexCo1-x Alloy Films,” Sol. St. Comm. v. 93, no. 12, p. 965 (1995) (hereinafter “Schreiber”), the disclosures of which are incorporated by reference herein. It has also been shown that alloys of these particular metals have damping parameters in the same order of magnitude as the constituent metals, see Schreiber; C. E. Patton et al., “Frequency Dependence of the Parallel and Perpendicular Ferromagnetic Resonance Linewidth in Permalloy Films, 2-36 GHz,” J Appl. Phys. v. 46, no. 11, p. 5002 (1975), the disclosures of which are incorporated by reference herein.
A magnetic switching device has been created with a switching threshold that is more stable than conventional switching devices. See Sun, U.S. Pat. No. 6,256,223, “Current-Induced Magnetic Switching Device and Memory Including the Same.” The magnetic switching device comprises two electrodes, at least one of which is ferromagnetic, and a single nanoparticle in between the two electrodes. The electrodes with the nanoparticle therebetween form an electrical switch. Switching of the device centers on the magnetic moment of the particle. Namely, a current is injected through the electrodes, and the nanoparticle therebetween, to overcome the magnetic moment of the particle and switch the device. Further, the magnetic switching device requires that a large diameter nanoparticle, on the order of several hundred angstroms, be employed.
Accordingly, it would be desirable to provide a magnetic material with beneficial properties for use in applications, such as magnetoelectronic devices, including but not limited to magnetic switching devices and magnetoresistive sensors. Beneficial properties include favorable magnetization properties, namely, increased damping and decreased coercivity. Favorable magnetization dynamics (i.e., increased damping) help the devices reach an equilibrium magnetic state, following a perturbation, in a more predictable and accurate manner. Lowering of coercivity allows for a more energy efficient switching and reduces unwanted heating. By being able to employ a magnetic material with these beneficial properties, more accurate and consistent devices may be produced.